As described in a pending U.S. patent application entitled COHERENT GEOLOCATION SYSTEM by Richard Schiffmiller, Henry Adler and Melvin Carroll filed on even date herewith, assigned to the assignee hereof and incorporated herein by reference, a coherent time-of-arrival (TOA) geolocation system is described. In one embodiment a collector aboard an aircraft or other vehicle is moved from one location to the next to collect data on which a TOA geolocation of a pulsed emitter such as a radar is made. However, to establish a long enough baseline for adequate location accuracy, it takes a considerable amount of time to fly the required baseline distance, sometimes 30–90 seconds. It would thus be desirable to be able to cut down the processing time for achieving geolocation.
By way of background, it is tactically important for a military aircraft that is overflying an enemy territory and detecting pulsed radiation from a radar to be able to locate the position of the radar so that the radar can either be destroyed, avoided or countermeasured. Two classes of time-based methods have been used in the past to geolocate a radar. The first utilizes time difference of arrival (TDOA) of radar pulses, measured either across two antennas of a single aircraft, or across multiple aircraft. The second measures the time of arrival (TOA) of a radar's pulses at a single platform in a non-coherent fashion by averaging data taken from a number of snippets called dwells. This system exploits the varying inter-pulse intervals due to movement of the platform from one position to another.
In both of the above cases the accuracy of the geolocation depends on the baseline between the collectors used to detect the emitted pulses or the distance the single collector moves during the geolocation measurement period. Note that the longer the baseline, the better will be the location accuracy. Prior multi-ship Time Difference Of Arrival systems, while useful, require that the same pulses be detected by more than one collector and that the collectors know which pulses on one platform correspond to which on the others. The latter requirement, if not met, can lead to ambiguous geolocations of the radar. The former requirement is even more severe. If the collectors do not detect the same pulses, the position of the emitting device cannot be accurately ascertained by the here-to-fore used methods.
Measuring the same pulses on multiple platforms is difficult to achieve. There may be physical obstructions that block a platform's line of sight to a radar so that pulses detected by one aircraft may not be detected by the others. Also, the collectors' receivers may not be tuned to the same frequency bands at the same time, and so will not detect the same pulses from the emitter. Finally, the collectors may not have the sensitivity to detect a scanning radar beam in its side lobes or back lobes, and the main beam of the radar may be illuminating only one collector at a time. As to TOA systems, prior time-of arrival systems that use non-coherent processing operate on snippets or dwells of data, with many snippets of data collected over many tens of seconds of flight in an attempt to establish a long baseline. This prior method measures the times of arrival associated with each snippet independent of the others and then averages the time-of-arrival results. This approach is called “non-coherent” processing as it does not exploit any possible long time uniformity or coherency across the snippets of data. Pulse data is coherent over a period only if there is some constancy in the radar emission process over that period, e.g., the pulse repetition interval (PRI) does not change.
As to PRI, typically the emitter's PRI is often purposely varied depending on the mode of operation or is inherently unstable over time. Thus, the reason for using short snippets of data in the past was to assure that the radar's PRI did not change over the measurement or that there are no gaps in receipt of the pulses, thereby assuring coherency at least over the snippet.
Note that when only a small snippet of data is considered the baseline associated with the data is exceedingly short. This means that any geolocation using the snippet alone will be unacceptably error-prone. Averaging the times of arrival in a dwell and using those average values to extend the baseline (non-coherent processing) typically requires more than 60 seconds of data to converge and does not produce geolocation results that are nearly as accurate as coherent processing.
To summarize, processing TOA data non-coherently involves averaging the time-of arrival results over each short snippet of data and finding a location for those values. Any coherent processing that is done occurs only over the short snippets involving short collection periods or dwells. Because of this, the resultant geolocation has limited accuracy despite averaging.